1. Field of the Invention
The present invention relates to a focus detection device for optical systems such as a camera, and more particularly, to a focus detection device which samples discrete areas of the image in order to determine focus.
2. Description of the Related Art
Cameras are known which sample rectilinear areas at the center of a view-finder in order to determine focus. Further, it is also known to use a central cross-shaped sampling area to determine focus. However, these sampling areas suffer, in that if the image has a linear object which lies parallel to the sampling area, it is difficult to detect, and thus the focus determination will be inaccurate. For example, when photographing artificial objects, or when the camera is held in an unusual manner, the object in the viewfinder often possesses a vertical or horizontal line structure which is parallel or nearly parallel to the edges of the finder frame. In this case, using a rectilinear focus area, parallel to the edges of the rectangular finder frame, it is impossible to detect or correct for focus. In general, lines which lie within 30.degree. of a rectilinear focus detection area are difficult to detect.
A further problem lies in reading narrow image lines lying nearly perpendicular to the focus detection area. This situation occurs due to the resolution of the CCD (Charge Coupled Device) sensor, i.e., the width of the individual elements. It is possible for a line to be slightly skewed, and still register as perpendicular to the focus detection area, decreasing the accuracy of the focus determination. Thus, there also exists a need for a focus detection device able to resolve narrow lines perpendicular to the focus detection area.
It is known to use a plurality of rectilinear focus detection elements. For example, Japanese Patent No. 63-11906 shows arranging rectilinear areas as the focus detection areas at numerous positions. However, these layouts, do not solve the problem of poor focus adjustment when the image contains line parallel to the elements.
It has also been proposed to place cross-shaped elements at each focus detection position. However, this has proven technically difficult, making it preferable to use a combination of cross and rectilinear shaped elements. For example, Japanese Patent No. 2-120712 shows arranging cross-shaped areas and rectilinear areas in a mixed manner at numerous positions. However, these layouts do not solve the problem. There still exists situations in which a linear image element will lie parallel to the detecting elements. Thus, there exists a need for discrete focus detection areas capable of detecting linear image elements parallel to the edges of the view-finder.
One possible remedial measure is to diagonally incline and arrange the rectilinear area to about 45.degree. relative to the finder frame. However, due to constraints stemming from spatial competition with the detection optical system, relative to adjacent areas according to the position in which the area is arranged, and from the hereinafter described inability to utilize large sub-mirrors, it is difficult to incline the detection area to approximately 45.degree..
Referring to FIGS. 3 (a) and 3(b) the light beams entering the camera are guided to the focus detection mechanism, arranged at the bottom of the mirror box in a single-lens reflex camera, by the sub-mirror 3 which receives the light through a semi-transparent area of the main mirror 2. On the AX axis, which crosses the main mirror surface where the extended surface of the sub-mirror intersects the main mirror, the size of the sub-mirror can be large. However, in the direction perpendicular to the AX axis, the light beams are constrained and focus detection can only be conducted in a narrow range E.
Furthermore, even if the positions where the focus detection areas are placed are identical, as where the lengthwise direction of the focus detection area is in the short edge direction of the sub-mirror (see FIG. 3(a) when one takes into account the spread of the detection light beam, the degree of freedom is small. In contrast, in the case where the lengthwise direction of the focus detection area is on the axis AX (see FIG. 3(b), the degree of freedom of the detection light beam is large.
Accordingly, in the case where a focus detection area is provided in the vicinity of the boundary line of the range E, it is necessary to make the angle formed by the boundary line of range E with the lengthwise direction of the focus detection area, i.e., the angle formed with the axis AX, as small as possible.